99 3.1L idle, coolant problems

[BernieMacDaddy]

i have a 99 cutlass with a 3.1L motor, i just put new headgaskets in it, along with all the other top end gaskets. when its idling in park it seems fine. but when i put it in gear, without moving anywhere, it idles up and down and kind of backfires and sputters. it didnt do this before i worked on it. also when its in gear coolant doesnt go in the motor, it shoots out of the overflow hose, it did do this before i worked on it and i still dont kno why, it has a new thermostat too. all the vacuum lines are conected properly and plug wires are too.

i was thinkin the radiator is blocked? it also has a new water pump. the idle im not sure tho? any ideas? it currently is straight piped becuase im replacin the catylitic converter, not enough back pressure? idk help please.